Chapter 7
Tests with High Lightning and Switching
Impulse Voltages
Abstract Lightning impulse (LI) over-voltages and switching impulse (SI)
over-voltages are caused by direct or indirect lightning strokes or by
switching operations in electric power systems, respectively. They cause
transient stresses to the insulations, much higher than the stresses due to the
operational voltages. Therefore, insulations must be designed to withstand LI
and SI over-voltages, and the correct design has to be verified by withstand
testing using LI test voltages, respectively, SI test voltages. This chapter deals
with the generation of aperiodic and oscillating LI and SI impulse voltages
and the requirements for their application in HV tests. Special attention is
given to the interactions between the LI/SI generator and the test object. The
deviations from the standardized impulse shape, e.g. by an over-shoot on the
LI peak, are analysed, and the evaluation of recorded pulses according to IEC
60060-1:2010 and IEEE St. 4 (Draft 2013) is described. This is completed by
the description of components and of the procedures for the correct mea-
surement of LI/SI test voltages. Also the measurement of the test currents in
LI voltage tests and the PD measurement at SI test voltages are included.
This chapter deals with the generation of aperiodic and oscillating LI and SI
impulse voltages and the requirements for their application in HV tests. Special
attention is given to the interactions between the LI/SI generator and the test
object. The deviations from the standardized impulse shape, e.g. by an over-shoot
on the LI peak, are analysed, and the evaluation of recorded pulses according to
IEC 60060-1:2010 and IEEE St. 4 (Draft 2013) is described. This is completed by
the description of components and of the procedures for the correct measurement
of LI/SI test voltages. Also the measurement of the test currents in LI voltage tests
and the PD measurement at SI test voltages are included.
W. Hauschild and E. Lemke, High-Voltage Test and Measuring Techniques, 285
DOI: 10.1007/978-3-642-45352-6_7, Ó Springer-Verlag Berlin Heidelberg 2014
286 7 Tests with High Lightning and Switching Impulse Voltages
7.1 Generation of Impulse Test Voltages
7.1.1 Classification of Impulse Test Voltages
A lightning stroke may cause—e.g. on a transmission line—a travelling wave of a
current pulse with a peak value ranging from few kiloamperes up to about 200 kA
(in very rare cases, even up to 300 kA). Investigations of Okabe and Takami on
UHV transmission systems (Takami 2007; Okabe and Takami 2009, 2011) con-
sidered peak currents up to 300 kA and a front duration in the range between 0.1
and 5 ls for the calculation of LI over-voltages (‘‘external over-voltage’’) based on
the surge impedance of the overhead transmission line, the grounding resistance of
a tower and the impedances of the involved components. Figure 7.1 shows the
resulting over-voltages for a GIS and a power transformer. Whereas at the GIS, the
shape and the peak of the over-voltage change with the front time of the current
pulse, the over-voltage at the transformer is not influenced by the front time of the
current impulse. In both cases, the front time of the over-voltage is in the order of
1 ls, but the over-voltage shows oscillations. IEC 60060:2010 defines all impulse
voltages with LI front timesT1 \ 20 ls being LI voltages. Standard LI test voltages
are aperiodic impulses. They are characterized by T1 = 1.2 ls and an LI time-
tohalf-value of T2 = 50 ls, abbreviation 1.2/50. In the case of Fig. 7.1 (Okabe and
Takami 2011), the LI over-voltages are quite well represented by standard LI test
voltages 1.2/50.
A switching process ‘‘on’’ or ‘‘off’’ in a power system causes an ‘‘internal over-
voltage’’ due to the excitation of internal oscillation circuit(s) formed by the
inductances and capacitances of the involved components of the system. The SI over-
voltages are also oscillating with one or several frequencies which are remarkably
lower than those of LI over-voltages (Fig. 7.2). All impulse voltages with front time
T1 [ 20 ls are defined in IEC 60060-1:2010 being SI voltages. Shape and param-
eters of SI test voltages shall not only represent SI over-voltages, but should also be
generated with the same test generator as LI test voltages (see Sect. 7.1.2). Their SI
time-to-peak of about Tp = 250 ls should meet a minimum breakdown voltage of
non-uniform air gaps with distances of about 5 m (Fig. 7.3, averaged characteristics
according to Thione 1983). Therefore, standard SI test voltages are aperiodic
impulses and characterized by Tp = 250 ls and a time to half-value of 2,500 ls,
abbreviation 250/2,500. They shall represent all kinds of SI over-voltages.
For on-site testing, also oscillating impulse voltages (OLI; OSI) are applied
(IEC 60060-3:2006). OLI and OSI test voltages can be generated with a generator
of efficiency factors about twice of those for LI and SI voltages (see Sect. 7.1.3).
Even if this is made for easier transportation and handling of the test system, it
should be mentioned that the used OLI and OSI test voltages represent quite well
the related over-voltages (compare with Figs. 7.1 and 7.2). Also the so-called
‘‘damped alternating voltage’’ (IEC 60060-3:2006; DAC) used for PD testing of
cable systems in the field—is an OSI voltage. OSI voltages can also be generated
by test transformers (see Sect. 7.1.4).
7.1 Generation of Impulse Test Voltages 287
3000
kV
2000
1000
0
-1000 1 2 3 4 µs 5
0
time t
voltage V
3000
kV
2000
1000
0
-1000 24 6 8 µs 10
0
Fig. 7.1 LI over-voltage caused by 200 kA LI current pulse of different front times
superimposed on the negative AC voltage peak at a GIS (above) and at a power transformer
(below)
time t / ms
voltage V
Fig. 7.2 Example for an SI over-voltage
288 7 Tests with High Lightning and Switching Impulse Voltages
breakdown voltage Vb Tp - Vbminimum characteristic gap distance d / m
2500
kV 13
2000 11
1500 9
7
5
3
1000
500 1
0 100 250 μs 1000
10 25 time-to-peak Tp
Fig. 7.3 SI breakdown voltage of long air gaps depending on the time-to-peak
Last but not least, it should be mentioned that in case of disconnector switching
in SF6-insulated systems (GIS), over-voltages faster than LI over-voltages are
generated. They are represented by fast front test voltages (FFV) and generated by
switching the disconnector in the GIS under test (see Sect. 7.1.5).
7.1.2 Basic and Multiplier Circuits for Standard
LI/SI Test Voltages
7.1.2.1 Basic RC Circuit
The operation of the basic circuit for impulse voltages shall be explained by its
equivalent circuit (Fig. 7.4a): When an impulse capacitorCi is charged via a
charging resistor Rc up to the DC breakdown voltage V0 of the switching gap SG,
the impulse voltage Vi is generated by the connected elements (Fig. 7.4b): Then,
the load capacitor Cl is charged via the front resistor Rf which forms the front
of the impulse voltage. At the same time, the impulse capacitor Ci is discharged
via the tail resistor Rt and forms the tail of the impulse voltage. The superposition
of both processes delivers a peak voltage Vip which is lower than the breakdown
7.1 Generation of Impulse Test Voltages 289
Fig. 7.4 Basic equivalent (a)charging charging switching front impulse
circuit for impulse voltage
generation. a Equivalent voltage resistor Rc gap SG (V0) resistor R f voltage
circuit diagram. b Potential
diagram VDC VSG Vi
impulse tail load
capacitor Ci resistor Rt capacitor Cl
(b) voltage V
VDCmax VSG
= V0
Vip
Vi
t0 tp time t
origin time-to-peak
voltage V0 of the switching gap. The relation between the two voltages delivers the
efficiency factor (also: utilization factor) of the ‘‘one-stage’’ (basic) impulse
generator:
g ¼ Vip \1; g ¼ gs Á gc: ð7:1Þ
V0
The efficiency factor g can be understood as a product of the efficiency gs
depending on the impulse shape and the efficiency gc depending on the circuit
parameters (Hylten-Cavallius 1988).The shape efficiency gs increases with the
relation between tail and front time of the impulse to be generated. When an LI
impulse voltage (1.2/50) shall be generated, the relation is, e.g. about 40, when an
SI voltage (250/2,500) is generated the relation is, e.g. 10 only. The circuit effi-
ciency gc depends mainly on the relation between impulse and load capacitor. The
larger the impulse capacitance Ci in relation to the load capacitance Cl, the higher
is the circuit efficiency gc. The overall LI efficiency factor is relatively high
(g & 0.85…0.95) and the SI efficiency factor remarkably lower, g & 0.70…0.80
only.
Note In addition of the circuit of Fig. 7.4a, a second basic circuit which has the tail resistor
not before, but after the front resistor, is sometimes discussed in textbooks. This circuit has
a lower efficiency factor. Therefore, it is not used practically and not discussed here.
290 7 Tests with High Lightning and Switching Impulse Voltages
The front of the impulse voltage is mainly determined by the time constant sf
and its tail by st
sf ¼ Rf Á Cl; st ¼ Rt Á Ci: ð7:2Þ
Usually, an impulse voltage generator is equipped with an impulse capacitor Ci
and a basic load capacitor Cl of fixed values. The front time can be adjusted by a
correctly selected front resistor Rf and the timetohalf-value by an appropriate tail
resistor Rt. With the fixed values of Ci and Cl, the maximum SI peak voltage is
only about 80 % of the maximum LI peak voltage.
Note The analytical calculation of the time parameters of impulse voltages and their
efficiencies is given in older textbooks. Today, the analytical calculation is replaced by
well-adaptable and commercially available software programs. This enables also the more
detailed consideration of the characteristics of the test object and the stray capacitances in
the test room and delivers more precise results.
The selection of the impulse capacitor Ci determines—together with the
maximum charging voltage V0max also the impulse energy of the generator:
Wi ¼ 1 Ci Á V02max: ð7:3Þ
2
Whereas the maximum charging voltage of a generator depends on the required
test voltages, the impulse capacitance must be selected according to the expected
total load (basic generator load plus test object load), to guaranty Ci ) Cl .
7.1.2.2 Multiplier RC Circuit
The basic circuit (Fig. 7.4a) is usually applied for students training and demon-
strations with voltages below 200 kV. For higher voltages, multiplier circuits
proposed by E. Marx in 1923 are applied (Fig. 7.5a, without the red short-circuit
bars):
The impulse capacitors Ci of all n stages are charged via the charging resistors
Rc which are connected in series to one column. When the charging resistors are
dimensioned correctly, it does not play any role that the charging resistance of the
highest stage is n-times larger than that of the lowest one, because the charging
time is selected long enough that all impulse capacitors are equally charged.
Today, a thyristor-controlled charging with a constant current up to a pre-selected
voltage V0 is used, at which the switching gap is triggered for breakdown. Now,
the impulse capacitors start to discharge via the tail resistors on each stage
(Fig. 7.5a: blue path). At the same time, the external load capacitance Cl (basic
load of a capacitive divider plus stray capacitances of the generator to ground plus
test object) is charged from the series connection of all impulse capacitors and
front resistors (green path). An impulse voltage generator of n stages (Fig. 7.5b)
charged with a DC voltage V0 delivers with the efficiency factor g the output
impulse voltage
7.1 Generation of Impulse Test Voltages 291
(a) SG Rf 3·C l
3·C I
Rc Rt
Rc (b) charging resistors front resistors
Rc Ci Rf SCB
SG
Rt
Ci
SG Rf
Rt
Ci Rf SCB
SG
Rt
Ci
Rc SG Rf
Rt
Ci Rf SCB 3·C l
SG trigger
generator
Rc
Rt
Rc Ci
DC charging
Cm>>Cl to measuring
voltage instrument
switching gaps tail resistors capacitors
Earthing resistance
and earthing switch
Fig. 7.5 Multi-stage impulse generator of n = 6 stages. a Multiplier circuit (explanations in the
text). b Impulse generator
Vin ¼ n Á g Á V0 ð7:4Þ
The term V0n max = n Á V0 max is called the cumulative charging voltage of the
generator and usually used as the rated voltage of the impulse test system because
V0n max [ Vin max one has to be careful with the valuation of rated voltages for
impulse test systems. It is always necessary to know the efficiency factor for all
impulse voltage shapes of interest for the calculation of the related output voltages
additionally.
For calculation of its circuit elements, a multi-stage generator (n stages, ele-
ments Rf; Rt, Ci; Cl) is usually transferred into an equivalent basic circuit with the
elements
292 7 Tests with High Lightning and Switching Impulse Voltages
Rf à ¼ n Á Rf ð7:5Þ
Rtà ¼ n Á Rt
Cià ¼ Ci=n
Clà ¼ Cl:
After the calculation of the circuit elements of the basic circuits, the Eq. (7.5) is
used for the determination of the multi-stage generator by re-transformation. The
thermal design of the resistors—especially of the front resistors—determines the
allowable impulse voltage repetition rate. The resistors are heated by the impulse
current, which is flowing in case of the impulse voltage generation and should
sufficiently cool down until the next impulse appears. A defined maximum tem-
perature of the resistors must not be exceeded.
The controlled safe triggering characterizes a generator of high quality. Usu-
ally, only the lowest stage is equipped with a so-called ‘‘trigatron’’, a three-
electrode arrangement (Fig. 7.6a). A small, battery-operated trigger device gen-
erates a voltage pulse of several kilovolts which causes a small trigger discharge at
a pilot gap. This discharge triggers the breakdown of the main gap of the lowest
stage. The trigger discharge delivers charge carriers and photons for the imme-
diate, fast breakdown process, if the field strength in the main gap is high enough.
This requires a certain minimum voltage, the so-called lower trigger limit
(Fig. 7.6b). If the voltage at the trigger gap is too high, a breakdown is caused
without triggering. This self-ignition delivers the upper trigger limit of the
charging voltage. The trigger range between the two limits (Fig. 7.6b) should be
as wide as possible. Usually, its width is between 5 and 20 % of the withstand
voltage of the non-triggered gap (upper curve). It depends on the design of the
trigatron, the energy of the trigger discharge and the height of the DC voltage at
the main gap.
The charging voltage and the trigger instant must be well controlled to guar-
antee safe triggering of the whole generator and to avoid ‘‘no-triggering’’ or self-
ignition without triggering. As soon as the lowest switching gap breaks down, an
over-voltage appears at the second stage, runs as a travelling wave through the
generator and shall cause the breakdowns of all further gaps. The over-voltages
must remain high enough to cause all necessary breakdowns. This depends on the
impulse shape to be generated (e.g. damping front resistors) and on stray capac-
itances to ground which increase the over-voltages, whereas longitudinal stray
capacitances reduce them (Rodewald 1969a, b). Based on such investigations,
additional trigger measures (e.g. supporting gaps and ignition capacitors) have
been introduced to maintain the height of over-voltages also for huge generators
(Rodewald 1971; Feser 1973, 1974). Generators with symmetric charging (Sect.
7.1.2.4, Fig. 7.7) allow a save triggering without these additional measures
(Schrader 1971).
The modular design of multi-stage generators is helpful for later extension to
higher voltages by additional stages. It enables also the parallel connection of
stages for higher impulse energy at lower voltages (Fig. 7.5.a, red short-circuit
7.1 Generation of Impulse Test Voltages 293
(a) grounded main trigger electrode HV main
electrode
with pilot gap electrode
connection for main gap d
trigger generator
(< 10 kV)
(b)
charging voltage
self-triggering
trigger
range
no triggering
main gap distance d
Fig. 7.6 Triggering of impulse voltage generators. a Trigatron spark gap. b Principle of the
trigger range
bars (SCB)) as they are, e.g. required for testing the low-voltage winding of power
transformers or medium-voltage capacitors. Also impulse test currents can be
generated by impulse voltage generators with parallel stages.
7.1.2.3 Consideration of the Inductances in the Circuit
Till now, all explanations have not considered the inductances in the impulse test
circuit which cannot be avoided. Inductances form oscillating circuits with the
capacitances and cause damped oscillations superimposed on the aperiodic pulses.
The damping depends on the front resistor. This is several 100 9X for SI voltages
and suppresses the oscillations completely. The more or less damped oscillations
294 7 Tests with High Lightning and Switching Impulse Voltages
(a) impulse generator HV connection test object chopping
Rc G Rf Li Le gap/divider
V DC C i Rt C lb C lt V LI
(b) strongly damped slightly damped
VLI
VLI
time t
Fig. 7.7 Inductances in the impulse voltage generation circuit. a Equivalent circuit diagram with
inductances. b Over-shoot superimposed on LI voltages (schematically)
and the ‘‘over-shoot’’ (only less than one period of the oscillation) are found at LI
voltages only, because the LI generator is equipped with front resistances of few
10 9X (Fig. 7.7). There are internal inductances of the generator and external of the
test object and its connections.
Internal inductances Li are those of the capacitors, the resistors and the con-
nections between them. For estimations, the inductance for 1 m of the loop (e.g.
green path in Fig. 7.5a) is about 1 lH. The reduction in the internal inductance of
a generator requires its compact design with a loop as short as possible. A good
generator should have an internal inductance of Li \ 4 lH per stage. Usually, the
user cannot influence the internal inductance of the generator easily. When only a
part of the stages is sufficient to generate the necessary voltage (so-called ‘‘part
operation’’), the loop should be short and should exclude the not-used part of the
generator and of the basic load (voltage divider). For very old generators, one
should check the inductance of the front resistors: The front resistors must be
designed with low inductance, which can be reached by a bifilar winding. This
means that two isolated, close together arranged wires are wound on a fibreglass
tube in opposite directions. The magnetic fields of the wires have opposite
directions and compensate each other to a remaining inductance which corre-
sponds to the length of the resistor tube. A second possibility is a resistor band
where the insulated resistance wires are woven into a fabric as a meander. Resistor
bands are commercially available. The inductance of resistors can also be reduced
when, instead of a single resistor, two or more parallel resistors are applied
resulting in the same resistance.
External inductancesLe are those of the test object (even if this mainly a
capacitance), the HV lead to the test object and the voltage divider as well as those
of the earth return. HV lead and earth return shall be especially very short and can
7.1 Generation of Impulse Test Voltages 295
often be influenced. With increasing LI test voltage, the distances between gen-
erator and test object become longer and oscillations and over-shoot cannot be
controlled in testing UHV equipment (see Sect. 7.3). Up to a certain degree, also
the inductance of the lead can be reduced by its selection. Table 7.1 gives some
indications of the inductances which depend also on the length of the connection.
Never a thin wire should be used for the HV lead or the ground return, because its
inductance is higher than those of copper foil of a width w C 10 cm or metallic
tube of a diameter d C 10 cm. A further reduction can be reached with a wider foil
or two parallel foils and spacers with a distance d in between. Also quite useful is
the application of the mentioned resistor bands as HV lead and external damping
resistor to the test object. To maintain the impulse shape, the internal front resistor
must be reduced, but the external resistor increases the damping efficiency
including the efficiency factor.
Over-shoot compensations can be designed as low-pass filters of L/C/R com-
binations (serial or parallel compensation unit) arranged inside the generator or
outside as separate components: Figure 7.8 shows the principles of the two
compensation units:
The series compensation unit (Fig. 7.8b; Wolf and Voigt 1997) prevents the
penetration of higher-frequency contributions to the load capacitance which
includes the test object. The series connection of compensation resistance Rc and
compensation inductance Lc must be adjusted to that of the front resistor Rf and
internal inductance Li. Also the compensation capacitor Cc has to be related to the
load capacitance Cl. The necessary adjustment covers a certain range of load cases,
but if fine tuning is required, the compensation unit must be adapted. For larger
impulse generators, the series compensation unit can be distributed to the different
stages of the generator (with elements of the stage voltage, e.g. 200 kV) and
without components of high-rated voltage (e.g. 3,000 kV).
The parallel compensation unit (Fig. 7.8a; Hinow and Steiner 2009; Hinow
2011) is always a separate unit which might be combined with a chopping gap and a
voltage divider to one compact unit (Fig. 7.9). Its adjustment has to consider the
natural frequency range caused by internal and external inductances. The efficiency
of the two principles is about the same. It seems that especially for LI testing of
UHV equipment the handling of compensation units becomes too time-consuming
and does not fit to the operation of an industrial test field. The problem can be easily
solved by increased damping due to larger front resistors, but this requires larger
tolerances for the front time of LI impulses (for details see Sect. 7.2.1)
7.1.2.4 Some Details of the Design of Impulse Voltage Test Systems
The LI/SI voltage test system (Fig. 7.10) includes the HV circuit consisting of the
HV generator which is optionally completed by an over-shoot compensation unit,
an HV chopping gap and a measuring system including an HV LI/SI divider (see
Sect. 7.5). The test object is also a part of the HV circuit, but later considered
under Sect. 7.3. Furthermore, it includes the control and measuring system, the
296 7 Tests with High Lightning and Switching Impulse Voltages
Table 7.1 Inductances to be assumed for HV leads
Connection by Single wire, Metal foil, Metal tube, Metal foil, w = 50 cm or 2
foils with spacer in between
length of connector d = 2 mm w = 10 cm d = 10 cm ( lH)
(m) ( lH) ( lH) ( lH) 0.40
0.84
1 1.37 0.70 0.59
10 1.83 1.26 0.96
(a) impulse generator HV connection overshoot test object
compensation chop. gap
Rc G Rf Li Le unit divider
VDC Ci Rt C lb C lt
Le
(b) Rc G R f Li
VDC Ci Rt Clb Clt
overshoot
compensation
unit
Fig. 7.8 Equivalent circuit diagrams of over-shoot compensation units. a Parallel compensation
unit. b Series compensation unit
switching cubicle with the thyristor controller and the DC voltage generator. In the
following, some characteristics of the main elements are given. For the generator,
see the above explanations.
Generator with symmetric charging: For larger impulse voltage generators, the
charging voltage per stage is usually 200 kV. Because of the limited rated voltage
of capacitors, usually two 100-kV capacitors are connected in series for 200 kV.
To guarantee identical charging of both capacitors of a stage, potential resistors Rp
must be arranged at the connection point of both capacitors (Figs. 7.11a and
7.12a). With a special circuit patented by Schrader (1971), a symmetric charging
with ±100 kV is applicable (Figs. 7.11b and 7.12b). This requires a charging unit
with symmetric output ±100 kV.
For each polarity, a separate column of charging resistors Rc, but no potential
resistors Rp are required. There are some advantages of the symmetric charging for
larger impulse test systems with two capacitors in series per stage: In the first line,
a stage with a short HV loop of low inductance can be designed (Fig. 7.12d).
7.1 Generation of Impulse Test Voltages 297
connected HV electrodes
parallel
compensation
(Fig.7.8 b)
capacitors for
voltage
control / HV
arm of divider
connected
earth
electrodes
adjustable gaps; LV arm of divider
Fig. 7.9 Parallel compensation unit in combination with voltage divider and chopping gap
HV lead
LI / SI
impulse
generator
thyristor HVDC HV arm test over- chop-
control- genera- voltage object shoot ping
divider compen. gap
ler tor
LV arm
voltage current earth connection
divider shunt current return
test bus
(Ethernet, Profibus, etc.)
main indu- digital LAN &
control strial recorder internet
PC connection
PLC
control & measuring system
Fig. 7.10 Components of an LI/SI test voltage system
298 7 Tests with High Lightning and Switching Impulse Voltages
(a) Charging resistors tail resistor Rt (b) 2 columns of charging
Rc front resistor Rf resistors Rc1 and Rc2,
no symmetry resistor
Impulse capacitors symmetry resistor
Rc2
Ci1 and Ci2 Rsym Ci2
Rc1
Cl
Rf
Rc Ci2 Ci1 Rsym
Rt
SG
Cl Rf
Ci1 Rt Rc2
Ci2
Rc Ci2 Rc1
SG
Ci1 Rsym Ci1
DC 200 kV Rt Cl
DC +100 kV DC -100 kV
Rf
Fig. 7.11 Typical circuits for impulse voltage generators. a Unipolar charging (e.g. 200 kV).
b Symmetric charging (e.g. ±100 kV)
The safe triggering of large generators with symmetric charging does not require
the additional measures mentioned above. As there are no voltage-dependent
circuit elements, the impulse shape (described by the time parameters) is inde-
pendent from the peak voltage (Fig. 7.13). Also the parallel connection of stages
for the generation of impulses with a higher energy is very simple.
Chopped lightning impulse (LIC) voltagesand chopping gap: An external over-
voltage in the power system is limited to the protection level by a lightning
arrester. This means the over-voltage is chopped and collapses to this protection
level. The duration of the voltage collapse is very short, its steepness very high.
Such steepness causes very non-linear stresses in equipment with windings
(power, distribution and instrument transformers, reactors, rotating machines). The
mainly stressed insulation at the HV terminals of the equipment must be designed
accordingly and verified by a test with chopped lightning impulse (LIC;
Fig. 7.14a) voltages.
An LIC test voltage is generated as an LI test voltage described above and then
chopped by a separate chopping gap. For LIC voltages up to about 600 kV, a usual
sphere-to-sphere gap can be used; for higher voltages, multiple chopping gaps
become technically mandatory (Fig. 7.14b and c). The voltage collapse of a
multiple spark gap is much faster than that of a single large sphere gap. The
chopping gap consists of in-series-connected sphere-to-sphere gaps, usually one
gap for one stage of the generator. One sphere of each gap is fixed and arranged at
a fixed insulating column. The other one—on suitable insulating support—is
7.1 Generation of Impulse Test Voltages 299
Fig. 7.12 Impulse voltage generators for unipolar and symmetric charging. a Generator with
unipolar charging (Courtesy of Haefely, Basel). b Generator with symmetric charging. c One-
stage of a multi-stage generator (inside view). d Cross section of a generator with symmetric
charging
moveable by a motor drive and can be adjusted for the relevant voltage value. The
parallel capacitor column controls the voltage distribution linearly. This column
might also be used as a damped capacitive voltage divider, which is usually a
separate component (see Sect. 7.5). The instant of the chopping can be triggered as
described above for the generator (Fig. 7.6). Also the combination with an over-
shoot compensation unit is applied (Fig. 7.9).
Electrodes for the HV components: An impulse voltage generator and the other
HV components require a sufficient clearance D from grounded or energized
300 7 Tests with High Lightning and Switching Impulse Voltages
front time (1.23±0.08)µs time-tohalf value(53.8±1.1) µs
0
-800
-1600
-kV
-2400
voltage V 0 0.5 1 1.5 2 2.5 0 20 40 60 µs
time
Fig. 7.13 Reproducibility of LI voltage shapes independent on peak voltage value
(a) (c)
(b)
modul 2
modul 1
Fig. 7.14 Chopped lightning impulse generation. a Chopped lightning impulse (LIC) voltage.
b Circuit of two modules of a multiple chopping gap. c Multiple chopping gap for 1200 kV (six
single gaps)
7.1 Generation of Impulse Test Voltages 301
objects in an HV test laboratory (Fig. 7.15a) to avoid breakdowns of the air gap
between the HV circuit and the surroundings. The necessary clearance depends on
the kind of pre-discharges which determine the breakdown process. The optimum
design of the electrodes of the HV components enables not only the correct
operation of an LI/SI test system but ensures also their minimum space in the test
laboratory.
Note This clearance should not be mixed up with the clearances for the test object
according to Fig. 2.1. The clearances there consider that the voltage distribution at the test
object is not influenced by the surroundings. Here, the operation of the generator shall not
be disturbed by undesirable discharges or even breakdowns.
At LI test voltages, the streamer discharge determines the breakdown voltage
of a non-uniform electric field in air. The electric field of an LI generator in an HV
test room is such a non-uniform electric field. Consequently, the specific break-
down voltage is equal to the voltage demand of streamers of about 5 kV/cm for
positive and about 10 kV/cm for negative polarity of the electrode with the larger
curvature. This means that for a 3 MV LI generator, the minimum clearance
should be 6 m (plus a certain safety margin of—say—20 %). The curvature of the
electrodes can be relatively small (Fig. 7.15b), because there is no need to avoid
the streamer discharges.
Note The strong polarity effect is typical for streamer discharges in air. There is no
remarkable polarity effect for internal insulation. If internal insulation, e.g. of a power
transformer, shall be tested with LI voltage, a flashover of the air-part of the bushing is
avoided when the test is performed at negative LI voltage.
At SI voltages, a combination of streamer and leader discharges determines the
breakdown voltage between the generator and the surroundings. The electrodes of
the HV components of the test system shall be designed in such a way that no
leader discharge appears, this means with larger radii (Fig. 7.15c). The effect of
enlargement of the distance to the surrounding is very week because of the low
leader gradient (about 1 kV/cm). Therefore, it is recommended to optimize the
electrodes of the HV components by a field calculation with the realistic condi-
tions of the test room. As a rough hint, the distance must be in minimum 20 %
larger than for LI voltage, and the surface field strength of the electrodes at SI
voltage should be about 20 kV/cm.
When a generator and related HV components are used for LI and SI genera-
tion, the electrodes are determined by the maximum SI test voltages, even when
they are about 25 % lower than the maximum LI test voltages. An optimum
utilization of a test area can be reached when a generator is moveable in the
laboratory, e.g. by air cushions. The design principles for outdoor generators
(Fig. 7.15d) are identical. They require also large electrodes for SI voltage gen-
eration, but under rainy conditions, the maximum output voltages must be
remarkably reduced.
302 7 Tests with High Lightning and Switching Impulse Voltages
(a) (b)
D
(c) (d)
Fig. 7.15 Electrodes for the HV components of LI/SI voltage test systems. a Necessary
clearance D around a LI/SI generator. b Test system only for LI voltage generation (2,000 kV),
c Test system 1,800 kV for LI/SI voltage generation at limited clearance to the ceiling. d Outdoor
test system 4,200 kV for SI and LI voltage generation (Courtesy of KEPRI, Korea)
7.1 Generation of Impulse Test Voltages 303
Control and measuring system: This—today usually computer-aided—sub-
system of an LI/SI test system (Fig. 7.10; see also Sect. 2.2) of an LI/SI voltage
test system enables the adjustment of the generator for the test voltage value and a
certain test procedure (see Sect. 7.4), the measurement of LI/SI voltages and of
related impulse currents (see Sects. 7.5 and 7.6). It is available for one, two or all
three following modes:
1. Manual operation with measurement and evaluation of LI/SI parameters: The
operator has to control the test system including adjustment of the voltages and
duration of the breaks between impulses, and the evaluation and presentation of
the test result (test record). The charging and triggering process must be con-
trolled. Control and measuring components are not connected to one system,
and this gap is filled by the operator. This traditional mode is very seldom
applied for industrial testing and research work, but applied for e.g. student’s
training.
2. Computer-supported operation and test result presentation: The test is per-
formed manually, but the precise adjustment of test voltages—this means that
of the distance of the switching gap as well as that of the charging DC volt-
age— and the test data presentation are overtaken by the system. Control and
measuring components are connected. This mode is applied for larger and
expensive test objects in industrial testing and for research work.
3. Automatic testing according to a pre-given test procedure by a computer con-
trol: The PC software for the test procedure is configured by the operator
before, the HV test itself is performed, evaluated and presented automatically.
Intervention of the operator is not necessary, but the test can be interrupted or
terminated at any time by the operator. This mode is applicable for testing of
very similar or even identical test objects in a larger scale or for statistical
investigations in research work.
The control system delivers the commands for switching the breakers on and
off, for adjusting the switching gaps of the generator for the pre-selected voltage
and for the appropriate charging voltage adjusted by a thyristor controller. Based
on the voltage measurement, the computer control checks that the voltage values
are within the pre-given sequence and tolerances. Based on the evaluation of the
voltage shape, breakdowns are recorded for the evaluation of the test. Also the
evaluation of the related currents might indicate whether a test has been successful
or has failed. The style of the test record depends fully from the intention of the
user.
Switching cubicle and DC rectifier unit: An LI/SI test system has a relatively
low power demand of some 10 kW. The switching cubicle contains the power
switch and the operation switch, the instrument transformers for supply voltage
and current measurement and protective equipment. The built-in thyristor con-
troller enables a constant-charging current output of the connected rectifier unit.
This DC rectifier unit is usually a doubler circuit (see Sect. 6.1.2 and Fig. 6.3) or
for symmetric charging, a half-wave rectifier with symmetric output (Fig. 6.2).
Depending on the rated power and energy of the impulse generator, the charging
304 7 Tests with High Lightning and Switching Impulse Voltages
voltage corresponds to the stage voltages (100–200 kV) and the charging currents
are between few 10 mA and some 100 mA. The duration of the charging which
determines the impulse voltage repetition rate depends on the total energy of the
generator and is usually between 10 and 60 s. For special application, also faster
charging processes and higher repetition rates can be realized.
7.1.3 Circuits for Oscillating Impulse Voltages
For factory testing with aperiodic LI voltages according to IEC 60060-1:2010 or
IEEE St.4:1995, the inductances in the circuit are disturbing elements, but a
defined inductance Ls in the circuit establishes an oscillating circuit of this series
inductance and the load capacitances Cl. This circuit is excited by the triggered
discharging of the impulse capacitors of the generator (Fig. 7.16a). The output
voltage is a damped oscillation around the discharge curve of the impulse
capacitances of the generator. The oscillating frequency is the natural frequency
f0 ¼ qffiffiffi1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi : ð7:6Þ
L Á Cl Á CiÃ
2p Á
s Cl þ CiÃ
For a generator with n stages, one has to apply Cià = Ci/n and RÃt = n Á Rt
(Eq. 7.5). The total load Cl = Clb ? Clt is the sum of the basic load and the test
object load. The fixed series inductance Ls replaces the front (damping) resistors.
According to IEC 60060-3:2006, impulse voltages with oscillations f0 [ 15 kHz
are considered as ‘‘oscillating lightning impulse (OLI) voltages’’ (Fig. 7.16a), such
with f0 \ 15 kHz as ‘‘oscillating switching impulse (OSI) voltages’’ (Fig. 7.17a).
The damping is determined by the losses in the circuit, for pure capacitive test
objects mainly by the tail resistors Rt of the aperiodic impulse. As these are higher
for OSI than for OLI voltages, OSI voltages show not only a lower frequency, but
also a larger damping (Fig. 7.17a).
Theoretically, the oscillating impulse voltage (OLI or OSI) can reach a peak
value which is twice the peak value of the relevant aperiodic impulse (LI or SI)
voltage. In practice, it reaches about 90 % of that value. The efficiency factors are
gOLI ¼ VOLI % 1:7. . .1:8 and gOSI ¼ VOSI % 1:3. . .1:4: ð7:7Þ
V0R V0R
As an example, Fig. 7.18 shows the remarkable influence of the test object (load)
capacitance on the efficiency factor and the time-to-peak. The high-efficiency
factors compared with those of the aperiodic impulse voltages are especially
important when mobile impulse test systems are required for the testing in the field.
Therefore, OLI and OSI voltages have been proposed for on-site testing (Kind
1974; Feser 1981), and meanwhile, they are standardized in IEC 60060-3:2006. For
more details, see Sect. 10.2.1.
7.1 Generation of Impulse Test Voltages 305
(a) (b)
(c)
voltage V
1 T1/T2 = 0.8/40 µs, 370 kHz
0
0 20 40 60 μs 80 time
1 T1/T2 = 20/100 µs, 16 kHz
0
0 20 40 60 s 80 time
Fig. 7.16 Oscillating lightning impulse voltages (OLI). a OLI test voltages. b Equivalent circuit
for oscillating impulse voltage generation. c 900 kV impulse test system for 850 kV LI and
1,600 kV OLI voltages (Courtesy of Siemens Berlin)
(a) (b)
voltage V
Tp /T2 = 20/1000; 15 kHz
0 500 1000 μs 1500
Tp /T2 = 400/4000; 1.25 kHz
0 1 2 3 4 ms 5 time
Fig. 7.17 Oscillating switching impulse voltages (OSI). a OSI test voltages. b 1,200 kV impulse
test for 900 kV SI and 1,600 kV OSI voltages
When the generator has to be designed for a maximum cumulative charging
voltage V0Rmax, the basic load capacitance and also the series inductance must be
able to withstand the maximum oscillating impulse voltage which is much higher
306 7 Tests with High Lightning and Switching Impulse Voltages
Fig. 7.18 OLI characteristic time-to-peak t p efficiency factor ηOLI
of an impulse voltage test 20 2.00
system (250 kV/5 kJ) μs
15 1.75
1.50
10
1.25
5
1.00
0
0 2 4 6 8 nF 10
total load capacitance
than the V0Rmax (Eq. 7.7). The insulation design of the basic load capacitance for
OLI and OSI voltages is practically identical, whereas that of the series inductance
is very different (compare Figs. 7.16c with 7.17b). For OLI voltages, a low
inductance is required which can be made easily. Contrary to that the OSI gen-
eration requires a much higher inductance. Now, stray capacitances must be taken
into consideration which would cause a non-linear voltage distribution along the
coil. To avoid that, a longitudinal voltage control by toroid electrodes is necessary.
The coil for OSI voltage is much longer, thicker and heavier than the one for OLI
voltage. Furthermore, it has been found that the benefit of OSI testing is low;
therefore, mainly OLI testing is applied (see Sect. 10.3.1).
It should be mentioned that also bipolar oscillating impulse voltages can be
generated based on impulse voltage circuits (Schuler and Liptak 1980). They
arranged the inductance in parallel to the load capacitance and applied the bipolar
OLI voltage for testing of rotating machines.
A special case of a bipolar OSI testing is applied for medium-voltage cables:
The cable is charged with a DC voltage and then discharged via a suited switch
(trigatron or semiconductor HV switch) in series with an inductance Ls and pos-
sibly a resistor Rd. The discharge causes a damped oscillation (Fig. 7.19). Under
the term ‘‘damped alternating voltage’’ (DAC) (IEC 60060-3:2006), this bipolarly
oscillating voltage is successfully used for diagnostic PD measurements on
medium-voltage cable systems, but the whole test stress for the cable is a long DC
ramp (duration in the order between 1 and 100 s) followed by the much shorter
DAC voltage (duration in the order of few 100 ms). The duration of charging and
the test frequency depends on the capacitance (length) of the cable, whereas
the damping of the oscillation depends on the losses in the circuit. Therefore, the
whole stress cannot be reproduced from cable test to cable tests. Occasionally, the
7.1 Generation of Impulse Test Voltages 307
(a)
+1
v(t)/Vt
0
3s (c) time t
-1 voltage v(t) / test voltage value Vt
time t +1
(b)
+1
v(t)/Vt 0
0
0.2 s
-1 -1
Fig. 7.19 Damped alternating (DAC) voltage. a A full DAC impulse (including the DC ramp for
charging). b The short oscillating part of the DAC impulse. c A sequence of DAC impulses as
occasionally used for withstand tests
voltage is used for withstand tests (Fig. 7.19c), but this cannot be recommended.
(for more details, see Sect. 10.3.2).
7.1.4 OSI Test Voltage Generation by Transformers
When an HV test transformer is excited by controlled discharging a capacitor bank
into its low-voltage (LV) side, this impulse causes an oscillation, which is trans-
formed to the HV side according to the transformer ratio (Kind and Salge 1965;
Mosch 1969). The schematic circuit diagram (Fig. 7.20a) is transferred with the
transformer ratio into an equivalent circuit (Fig. 7.20b), which is used for calcu-
lations of the shape and frequency of the OSI voltage (Schrader et al. 1989).
Shape and frequency are determined by the stray inductance of the transformer
(Ls) and the series connection of the bank capacitance (CB) with the transferred
load capacitance (CiÃ)
C ¼ CB Á Clà : ð7:8Þ
CB þ ClÃ
The impulse parameters can be influenced by adjustable elements, an induc-
tance Lr and a damping resistor Rp. With the total capacitance C and the total
inductance L = Ls ? Lr, one gets the frequency and the time-to-peak:
308 7 Tests with High Lightning and Switching Impulse Voltages
(a) capacitor thyristor tail adjustable test total
bank switch resistor inductance transformer load
CB Rp LR L0 Cl (c)
OSI voltage V OSI
VDC V OSI
(b) LR L0 time t
Cl* VOSI
VDC
Fig. 7.20 Generation of unipolar OSI voltages by transformers (Schrader et al. 1989). a
Schematic circuit diagram. b Equivalent circuit. c Unipolar OSI voltage
f ¼ p1ffiffiffiffiffiffiffiffiffi and Tp ¼ 1 ¼ pffiffiffiffiffiffiffiffiffi ð7:9Þ
2p L Á C 2f p L Á C:
The output (Fig. 7.20c) is a unipolar OSI voltage with frequencies of 100 up to
1,000 Hz; this means with time-to-peak Tp [ 500 ls.
Bipolar OSI voltages can be generated with a modified circuit (Fig. 5.21a and
b) (Schrader et al. 1989). The load capacitance is charged in the same way as for
unipolar OSI voltages, but when the first peak is reached, a short-circuit switching
by a thyristor causes the bipolar OSI voltage at the HV output (Fig. 7.21c). The
capacitor bank is not any longer involved in the oscillation.
Even under optimum conditions, shorter time-to-peak than mentioned above
cannot be generated because of the value of stray inductances of test transformers.
In many cases, these times are remarkably longer. When a transformer cascade of
three stages shall be used for OSI generation, the stray capacitance depends on the
kind of feeding (Fig. 7.22) (Schrader et al. 1989). Feeding into the primary side of
the lowest transformer means highest stray inductance and lowest frequency, say
100 Hz. When feeding is applied to the middle of the cascade (tertiary winding of
the second transformer), the frequency increases by a factor of two (200 Hz).
When feeding is realized into the tertiary windings of all three transformers, the
stray inductance decreases to 1/40 compared with case a), and consequently, the
frequency increases to more than 600 Hz. This principle has been applied to the
mentioned 3-MV cascade transformer (Frank et al. 1991), (Figs. 3.15 and 7.23).
On each stage, there is a capacitor bank with a rectifier unit. The DC voltage is
generated on the stages from a low-frequency AC voltage supplied via the
7.1 Generation of Impulse Test Voltages 309
(a) capacitor thyristor tail adjustable test total time t
bank switches resistor inductance transformer load
CB Th1, Th2 R p LR L0 Cl (c)
OSI voltage VOSI
VDC VOSI
(b) RP LR L0 TH 2 opedrates
TH2 time-to-peak
VDC VOSI
Fig. 7.21 Generation of bipolar OSI voltages by transformers (Schrader et al. 1989). a
Schematic circuit diagram. b Equivalent circuit. c Bipolar OSI voltage
Fig. 7.22 Feeding modes of (a)
a three-stage cascade
transformer for OSI voltage
generation. a Single feeding
into the primary winding of F
the lowest transformer. b (b)
Single feeding into the
tertiary winding of the middle
transformer. c Triple feeding
into the tertiary winding of
each transformer F
(c)
FFF
windings of the transformer in a special mode. The three capacitor banks are
discharged each into the tertiary winding of one transformer at the same time. This
enables the generation of OSI voltages up to 4.2 MV. The leader discharges in air
generated by this extremely high OSI voltage are very similar to natural lightning
(Hauschild et al. 1991).
310 7 Tests with High Lightning and Switching Impulse Voltages
capacitor banks on all stages
Fig. 7.23 3-MV transformer cascade with OSI attachment and triple feeding mode
Fig. 7.24 Time voltage V
characteristic of a VFF 50 ns
voltage
0 500 1000 ns 1500 time
7.1.5 Circuits for Very Fast Front Impulse Voltages
VFF over-voltages are generated by switching GIS disconnectors and consecutive
reflections in the GIS busbars, by steep LI voltage breakdowns of the insulation of
overhead lines or by the operation of a lightning arrester. Similar voltages are
expected in case of a nuclear explosion (EXO-EMP). They might be characterized
by an oscillating impulse with a first front of some 10 ns up to few 100 ns and
superimposed contributions of higher frequencies (Feser 1997). Figure 7.24 shows
a typical example of a VFF voltage (CIGRE WG 33.03 1998)
VFF voltage testing of GIS is performed by defined switching of the discon-
nectors (IEC 61259:1994). There is no horizontal standard for VFF test voltages.
For research and development of components which might be stressed by VFF
over-voltages in service, VFF impulse voltages are usually generated by a Marx
7.1 Generation of Impulse Test Voltages 311
(a) (b)
voltage V
LI voltage VVFF VLI
generator VLI
VVFF
steeping circuit time t
Fig. 7.25 Generation of VFF test voltages. a Equivalent circuit. b Potential diagram
Fig. 7.26 Principle circuit LI voltage
for VFF investigation of gas generator
insulation
bushing
steeping circuit: configuration under test
capacitance to earth against enclosure
Impedance (L or R) for gap for fast
VFF voltage shaping switching
impulse voltage generator with a connected steeping circuit (Kind and Feser 1999)
(Fig. 7.25), consisting mainly of a capacitor and a fast sphere gap with com-
pressed-gas insulation and high breakdown field strength. This gap is connected in
series with the test object and enables front times in the order of few 10 ns. An
impulse voltage generator without steeping circuit, operating without front resis-
tors, can generate impulse voltages with front times down to about 100 ns.
The latest development of UHV equipment has directed the attention to the
behaviour of compressed-gas insulation under VFF stress (Ueta et al. 2011; Wada
et al. 2011). For VFF voltage generation, a metal enclosed, compressed-gas-
insulated steeping circuit is applied (Fig. 7.26). In the field compartment before
the series gap, an impedance (resistor or inductor) is arranged to generate different
superimposed oscillations. When its position is changed related to the gap different
superimposed oscillations appear (few MHz to 20 MHz). The test object is inside
of the same metal enclosure.
312 7 Tests with High Lightning and Switching Impulse Voltages
7.2 Requirements to LI/SI Test Systems and Selection
of Impulse Voltage Test Systems
The preceding subsections have shown that a wide variety of impulse voltage
shapes can be generated. For research work, development and even diagnostic
testing, this variety can be used. But for quality testing, impulse voltages shall be
applied which represent external (lightning) and internal (switching) over-voltages
being reproducible within certain tolerances. Requirements for these test voltages
are given in standards like IEC 60060-1:2010 or IEEE Std. 4 (Draft 2013) and will
be explained in the following.
7.2.1 LI Test Voltage and the Phenomenon of Over-shoot
7.2.1.1 Requirements of IEC 60060-1 and IEEE Std. 4 to Standard LI
Voltages 1.2/50
The parameters of a LI test voltage shall be evaluated from the so-called ‘‘test
voltage curve’’ which is based on an equivalent processing of the recorded impulse
voltage with a possible over-shoot of different magnitudes and frequencies f (in
MHz) using the test voltage function
kðf Þ ¼ 1 þ 2:2 1 2=MHz2 : ð7:10Þ
Áf
This empirically determined function (Fig. 7.27) shall represent that an over-
shoot of long duration (low frequency) has a stronger influence on the breakdown
voltage of insulation than one of short duration (high frequency). This is the well-
known breakdown voltage—breakdown time characteristic of insulations. As a
first step of introduction of this new type of evaluation, IEC 60060-1:2010 rec-
ommends the application of Eq. 7.10 to all types of insulation, for the necessary
improvement of the method see Sect. 7.2.1.2. The determination of the test voltage
curve Vt(t) from the recorded curve Vr(t) shall be made in the following steps
(Fig. 7.28, for more details, see Annex B of IEC 60060-1: 2010):
1. Determine the base curveVb(t) as an estimate of the exponential function with
the parameters V0, s1 and s2:
VbðtÞ ¼ V0 Á eðt=s1Þ À eÀðt=s2Þ :
ð7:11Þ
The base curve (Eq. 7.11) represents the recorded curve without over-shoot and
shall be characterized by its peak value VB, whereas the full recorded curve is
characterized by its extreme value VE (Fig. 7.28a).
7.2 Requirements to LI/SI Test Systems 313
k-factor value 500 kHz
1
mean curve determines
0.8 breakdown
0.6
0.4 peak value determines breakdown
0.2
0
1 10 100 1000 kHz 10000
frequency of overshoot
duration of overshoot
Fig. 7.27 Test voltage function (IEC 60060-1:2010) with empirical data and limit value of IEC
60060-1:1989
(a) Recorded curve (b)
Base curve
VU VUEe 1
β
k-factor curve
VUBb
0.8
filtering of residual
0.4
0 Residual curve 0.2
t
(c) division of recorded curve
f/MHz = 0.10 0.32 1.0 3.2 10
U Recorded curve
0
V VUEe
VUBt (d) superposition
result V
VTt
Test voltage curve
VUBb
Test voltage presentation Base curve
Filtered residual
0 0 t
t
test voltage curve
test voltage curve for parameter
evaluation – except of overshoot β!
Fig. 7.28 Determination and presentation of the test voltage curve. a Recorded curve, base curve
and residual curve. b Test voltage function. c Base curve, filtered residual curve and test voltage
curve. d Presentation of test voltage curve and recorded curve
314 7 Tests with High Lightning and Switching Impulse Voltages
2. Find the residual curve as the difference between the recorded curve and the
base curve (Fig. 7.28a):
VRðtÞ ¼ VrðtÞ À VbðtÞ: ð7:12Þ
3. Use a digital filter with a transfer function (amplitude–frequency response)
equal to the test voltage function H(f) = k(f) (Eq. 7.10, as described in detail in
IEC 60060-1:2010, Annexes B and C) and use it for filtering the frequency
spectrum VR(F) of the residual curve. The result is the filtered residual curve in
the frequency domain
VRFðf Þ ¼ kðf Þ Á VRðf Þ; ð7:13Þ
and—after re-transformation to the time domain—the filtered residual curve
VRF (t) (Fig. 7.28b).
4. Superimpose the filtered residual curve on the base curve to get the test voltage
curve (Fig. 7.28d):
VtðtÞ ¼ VbðtÞ þ VRFðtÞ: ð7:14Þ
5. In a presentation of the result, both—the recorded curve and the test voltage
curve—shall be shown (Fig. 7.78c).
Note It should be mentioned that the handling of the zero-level problem is not considered
here. For the zero level and the details of the implementing, the evaluation software, see
IEC 60060-1:2010, Annexes B and C and IEC 61083-2:2013, see for the filter curve also
Lewin et al. (2008).
It should be mentioned that in addition to the computer-aided evaluation, also a
manual calculation of the test voltage value VT is described in IEC 60060-1:2010
(Annex B.4) and by Berlijn et al. 2007. This procedure considers not the whole
frequency spectrum of the residual curve, rather its corresponding value k(fos) at the
single main frequency fos of the over-shoot which is simply multiplied with the
maximum of the residual voltage VRmax(t). The result is superimposed on the esti-
mated base curve to get the test voltage value VT. In contrast, the filtering (Eq. 7.13)
works also when the over-shoot is the result of a mixture of frequencies and when
noise signals of higher frequencies are superimposed on the recorded curve.
In case of a smooth recorded curve (Eq. 7.11) with VE = VB, one gets
VR(t) = VRF(t) = 0 because of k(f) = 1. IEC 60060-1: 2010 requires that all
parameters of the LI test voltage are evaluated from the test voltage curve. When an LI
test voltage fulfils the following requirements, it is a standard LI test voltage 1.2/50:
The test voltage valueVT is the maximum value of the test voltage curve (Figs. 5.
78c and 7.29). In an LI voltage test, the required test voltage value must be
adjusted with a tolerance of ±3 %.
The front timeT1 is a virtual parameter defined as 1.67 times the interval between
the instants when the impulse voltage is 30 and 90 % of the test voltage value (A
and B in Fig. 7.29). The front time T1 = 1.2 ls has a tolerance of ±30 %, this
means the real front time has to be within (0.84–1.56) ls.
7.2 Requirements to LI/SI Test Systems 315
Fig. 7.29 Parameter normalized voltage V/Vt peak voltage Vp = test voltage value Vt
definition for full LI test
voltages 1
0.9
0.5 time
0.3
0
time-to-half value T2
front time T1 = 1.67·TAB
TAB
virtual origin 01
not measurable true origin 0
Note For test objects of high capacitance as cables or capacitors, the upper tolerance limit
might be significantly enlarged to 5 ls or even more. Also for UHV equipment, an upper
tolerance limit in the order of 2.5 ls is under discussion.
The time to half-valueT2 is a virtual parameter as the time interval between the
virtual origin which is the intersection between the time axis and the straight line
drawn through the points A and B in Fig. 7.29, and the instant when the voltage
crosses the half of the test voltage value (Fig. 7.29): It is required T2 = 50 ls with
a tolerance of ±20 %, this means the real value has to be within (40–60) ls.
The relative over-shoot magnitudeß is the difference between the extreme value of
the recorded curve and the maximum of the base curve related to the extreme
value.
b ¼ VE À VB 10 % ð7:15Þ
VE
The latest draft of IEEE Std. 4 recommends an over-shoot of 5 %, but allows an
increase to 10 % for reasons ‘‘to allow waveforms accepted by the historical’’
smooth curve ‘‘over-shoot method’’ (IEEE Std. 4—1995 and IEC 60060-1:1989).
Quality tests require in addition to full LI test voltages also chopped LI test
voltages (LIC) which represent the stress of the insulation after a protecting device
(e.g. an arrester or a protection gap) has operated. An LIC voltage is also caused by
any breakdown in the HV circuit, but in the following, only controlled breakdowns
with a chopping gap will be considered (see Sect. 7.1.2.4).
The LI voltage can be chopped in the front (Fig. 7.30a) or on the tail
(Fig. 7.30b). The instant of chopping is defined as the intersection of the line
through the points C (0.7 VCH) and D (0.1 VCH) with the voltage level immediately
316 7 Tests with High Lightning and Switching Impulse Voltages
(a) (b) (c)
normalized voltage V/Vt B B VCH B
1 C
C
0.9 0.7·VCH deviation
≥ 0.05·T1
0.7
0.3 A D A
A
0.1
0 TC D0.1·VCH
T1 T1
TC
undershoot TC - time to chopping T1 - front time
Fig. 7.30 Chopped lightning impulse voltages. a Front-chopped LIC voltage. b Tail-chopped
LIC voltage. c Linearly rising front-chopped impulse
before the collapse. The time to chopping TC is the interval between the virtual
origin O1 and the instant of chopping. The duration of the voltage collapse TCO is
defined as 1.67 times the time interval between the points C and D. The virtual
steepness SC of the chopping is calculated with the voltage VCH at the instant of
chopping and the duration of the voltage collapse TCO:
SC ¼ VCH : ð7:16Þ
TCO
Whereas IEC 60060-1:2010 specifies only a tail-chopped LIC voltage of
TC = 2–5 ls (Fig. 7.30b), the IEEE Std. 4 specifies also a standard front-chopped
LIC voltage of TC = 0.5–1.0 ls (Fig. 7.30a).
Note According to the IEC opinion, a front-chopped LIC voltage is not required for testing
objects with windings, because the tail-chopped LIC voltage causes a higher steepness and
consequently a more non-uniform voltage distribution in the test object. The traditional
IEEE opinion considers more the representation of high over-voltages limited by pro-
tection devices. It seems that in a future version of IEEE Std. 4, the IEC practice will be
applied, too.
A linearly rising front-chopped LIC voltage is a voltage rising with an
approximately constant steepness until it is chopped at a voltage VE. The linearly
rising front-chopped LIC voltage is mainly applied in the test practice according to
the IEEE standards. It is defined by the extreme value VE, the front time T1 and the
steepness
SF ¼ VE ð7:17Þ
T1
7.2 Requirements to LI/SI Test Systems 317
The voltage increase is considered to be approximately linear from 30 % up to
the instant of chopping. The tolerance of the steepness is characterized by a band
of ±0.05 Á T1 from the mean line through AB (Fig. 7.30c). The parameters of
linearly rising LIC voltages are not specified in the horizontal standards, but in the
relevant apparatus standards.
The evaluation of tail-chopped LIC voltages can be made with a method
adapted to the k-factor calculation. For this, two records are needed, one of the
tail-chopped LIC voltage from the performed test and one full reference LI voltage
on lower voltage without changing the set-up of the HV test circuit (except of the
switching gaps and the charging voltage of the generator) and the measuring
system. The reference LI curve is used for the determination of the base curve. The
recorded LIC curve is treated with that base curve similar as described above. For
more details, see IEC 60060-1:2010 (Annex B.5).
The problem of over-shoot does not appear for front-chopped impulses, they
can be evaluated as shown in Fig. 7.30a.
7.2.1.2 Situation and Future of the Treatment of Over-shoot
The evaluation method according to the IEC and IEEE standards as described
above is an important first step into the direction of a physically correct evaluation
of LI test voltages with over-shoot, but it is also a compromise between new ideas
and traditional thinking. Therefore, it shall be tried to explain in the following the
possible directions of the further improvement of the k-factor method. Let us
consider the new evaluation method in comparison with that of IEC 60-1:1989
(Fig. 7.27):
As considered in Sect. 7.1.2.3, the inductance and capacitances in the circuit
may cause oscillations which are damped by the resistive losses in the circuit. The
oscillations have remarkable influence on the breakdown behaviour when they
appear in the region of the peak and increase the peak value of the LI test voltage.
In case of a strong damping, the oscillation is reduced to a single half-wave, which
is called ‘‘over-shoot’’. In the following, the term ‘‘over-shoot’’ shall also include
oscillations of lower damping.
The previous version of IEC 60-1:1989 tolerated oscillations and over-shoot up
to 5 % of the smooth peak. If their frequency ‘‘is not less than 0.5 MHz or the
duration of the over-shoot not more than 1 ls, a mean curve should be drawn …for
the purpose of measurement’’. The test voltage value was the peak of the recorded
curve for over-shoot frequencies f \ 0.5 MHz; for f [ 0.5 MHz, it is the maxi-
mum of the drawn mean curve. There was no rule how to estimate the mean curve.
This abrupt change of the evaluation at 0.5 MHz (Fig. 7.27) is physically wrong,
causes an error of up to 5 % at that frequency, a certain arbitrariness for the
operator or for provider of evaluation software. Therefore, a change had been
urgent. On the mid of the 1990s, the CIGRE-Working Group 33.03 and a related
European research project started experiments on the influence of the over-shoot
(e.g. Garnacho et al. 1997, 2002; Berlijn 2000; Simon 2004). A combined voltage
318 7 Tests with High Lightning and Switching Impulse Voltages
(see Sect. 8.1.1) of a smooth LI voltage and an oscillating short impulse were
applied to insulation samples of air, SF6, oil-impregnated paper and polyethylene.
The test voltage values of test series were usually limited up to 200 kV. The
experiments delivered 50 % LI breakdown voltages for the breakdown of the
impulse with over-shoot (extreme value VE), for the smooth standard impulse
(peak value VLI) and enabled the determination of a well-defined base curve (Eq.
7.11; maximum VB). For each sample, the results at different over-shoot fre-
quencies have been combined as the frequency-depending test voltage factor (test
voltage function)
kðf Þ ¼ VLIðf Þ À VBðf Þ : ð7:18Þ
VEðf Þ À VBðf Þ
A clear decrease in the test voltage factor with increasing frequency has been
found (Fig. 7.27, measuring points), but the dispersion of the results was so large,
that no clear influence of the different types of insulations has been identified.
Therefore, a common k-factor curve has been evaluated (Fig. 7.27 and Eq. 7.10),
which is overtaken into the standards (see Sect. 7.1.2.1).
The results of the LI parameter evaluation according to the valid IEC 60060-1:2010
differ from those according to IEC 60-1:1989. Even if the new evaluation delivers
physically better results, the differences may have certain consequences for design and
testing of equipment. The results of the evaluation of numerous LI voltages (e.g. of
IEC 61083-2: 2013) according to the old and the new procedure (Table 7.2) show the
consequences. If there is an over-shoot with f \ 0.5 MHz, an up to 3 % higher LI test
voltage would be necessary now. The front time would become shorter and the time to
half-value longer. For f [ 0.5 MHz, an up to 6 % lower LI test voltage can be applied
now, the front time increases and the time to half-value decreases. These results show
the tendency, but they include not only the differences in the procedures, but also the
uncertainties caused by the software. Further comparisons of the old and the new
method are published by Pfeffer and Tenbohlen (2009).
An unexpected result was found for the values of the over-shoot which became
usually higher according to IEC 60060-1:2010 than according to the old version.
The reason is the missing rule for the old ‘‘mean curves’’ which got higher maxima
by the old ‘‘user-friendly’’ software than the well-defined ‘‘base curves’’ (Eq. 7.11)
by the new software. Also the definition of the over-shoot (Eq. 7.15) is physically
not correct. It does not consider the duration of the over-shoot (Hinow et al. 2010),
because the extreme value VE of the recorded curve is the reference value. An
over-shoot definition which considers the duration could follow a German proposal
to TC 42 when the over-shoot magnitude would be defined from the test voltage
curve—as the other parameters of LI voltage, too (Hinow et al. 2010):
bà ¼ VT À VB ð7:19Þ
VT
The comparison of the two definitions (Fig. 7.31) shows that ß* delivers always
lower values than ß. For an over-shoot frequency of f \ 0.5 MHz, ß* is typically
7.2 Requirements to LI/SI Test Systems 319
Table 7.2 Differences of LI parameter evaluation according to IEC 60060-1:2010 and IEC 60-
1:1989
Parameter Over-shoot frequency Over-shoot frequency
f \ 0.5 MHz f [ 0.5 MHz
Test voltage value 0…-3 % +2 %…+6 %
(V2010 - V1989)/V1989 0…-6 % 0…+15 %
Front time
(T1 2010 -T1 1989)/T1 1989 0…+5 % -4 %…7 %
Time to half-value
(T2 2010 -T2 1989)/T2 1989 Independent on the frequency
Over-shoot -10 %…+40 %
(ß2010 - ß1989)/ß1989
relative overshoot β* (equation 7.19) β*= β
25
% β* at
≈ 500 kHz
20 β* for f > 500 kHz
shorter duration
β* for f < 500 kHz
longer duration
15
10
5
0 5 10 15 20 % 25
relative overshoot magnitude β (IEC 60060-1:2010)
Fig. 7.31 Comparison of the over-shoot magnitudes of b (IEC definition Eq. 7.15) and ß*
(Eq. 7.19)
higher than for the case f [ 0.5 MHz. With respect to the duration of the over-
shoot, this is a plausible characteristic.
A further point is the dependence on the frequency. The frequency is estimated
from the duration of the over-shoot (Garnacho et al. 1997), but the breakdown
process is influenced by the available time according to the well-known break-
down voltage—breakdown time characteristic (see Sect. 7.3). This characteristic
can be described by a statistical time-lag followed by the formative time-lag. The
latter can be described by the formative voltage–time area (Kind 1957) which
might be also applied to the over-shoot treatment (Hauschild and Steiner 2009;
Garnacho 2010; Ueta et al. 2011c). Then, the over-shoot will be characterized by
320 7 Tests with High Lightning and Switching Impulse Voltages
(a) (b) (c)
voltage V V V
VE VE Tt = Tt1 + Tt2 VE
Vx Vx VT
Tt Tt1 Tt2 Tt
time t t
t
Fig. 7.32 Definition of over-shoot in conjunction with the formative time model. a Example for
an aperiodic over-shoot using a percentage of extreme value VE. b Example for oscillating over-
shoot using 0.9 Á VE. c Example for oscillating over-shoot using the test voltage value VT
the voltage–time area above a certain voltage value VX (Fig. 7.32). This might be a
certain percentage of the extreme value (Fig. 7.32 a, b) or the test voltage value VT
(Fig. 7.32c). Then, for example, the relative over-shoot magnitude would be cal-
culated by:
1 ZT
VT Á
b ¼ Tt Á ðVtðtÞ À VT Þdt; ð7:20Þ
with the over-shoot duration: Tt ¼ Pn tiðVi ! VT Þ:
i¼1
A limitation of ß° would mean a limitation of the formative voltage–time area
which considers the really acting stress combination of voltage and time.
When an over-shoot definition related to the duration of the over-shoot is
applied, it seems to be appropriate to apply also the test voltage function
depending on the duration. The duration of an aperiodic over-shoot is related to the
frequency by Tt & 0.5/f, and one can derive from Eq. (7.10), the new test voltage
function
4 Tt2 ls2
kà ðTt Þ ¼ 4Tt2 ls2 þ 2:2 : ð7:21Þ
This function (Fig. 7.33) would not only improve the physical understanding. It
has a linear scale with a direct relation to the time parameters of the LI voltage.
The case Tt = 0 means no over-shoot. The determination of the duration is even
simpler than that of the frequency. It would also consider the case that the
oscillations are only slightly damped (Fig. 7.32b), and the second peak contributes
to the formative voltage–time area. Instead of using the test voltage function of
IEC 60060-1:2010 (Eq. 7.10), a different definition of the relative over-shoot
magnitude can be used—e.g. as that of Eq. 7.20.
7.2 Requirements to LI/SI Test Systems 321
Fig. 7.33 Test voltage k-factor value
function depending on over-
shoot duration (Eq. 7.21)
µs Tt
MHz frequency f
The test voltage function for higher voltages and different insulation samples is
under investigation till now, e.g. by Garnacho (2010), Hinow with TU Cottbus
(2011), Ueta et al. (2011b). The present function (Eq. 7.10) is based on experi-
ments of small samples and voltages mainly VT \ 200 kV. Therefore, the break-
down is quite fast and the assumption to have a certain average characteristic
seems to be not correct. It must be shifted (Fig. 7.34)—to the left (lower fre-
quencies) for large insulation and/or relatively slow breakdown processes as for
long air gaps or larger transformer insulation used for UHV transmission (Tsuboi
et al. 2011, 2013; Ueta et al. 2012b) and only a bit to the right (higher frequencies)
for small and compact insulation of very fast breakdown processes (SF6; solids,
vacuum, etc.), but till now, no final result of that international research work is
available. It can be assumed that different characteristics will be found for different
electric fields, different insulation materials and possibly even for different over-
shoot magnitudes. Also the remarkable uncertainty of the determination of the test
voltage function must be taken into consideration. Possibly, all available experi-
mental results should be used to find one test voltage function for all parameter
evaluation according to a future standard IEC 60060-1.
The fitting method for extracting the base curve as described above (Eq. 7.11) is
also subject of ongoing investigation, e.g. (Satish and Gururaj 2001; Kuan and
Chen 2006; Ueta et al. 2011a, b, c, d, 2012a, b; Garnacho et al. 2013). The
introduction of a new evaluation method for the base curve could cause differences
in the LI parameter evaluation. Contrary to the test voltage function, there seems to
be no urgent need for the introduction of a new method for the determination of the
mean curve. First, it must be shown that the present method is insufficient for
practical cases.
Last but not least, there are many discussions to reduce the over-shoot in LI
testing by the over-shoot compensation (see Sect. 7.1.2.3). A more effective (and
cheaper!) way is the increase in the upper tolerance limit of the front time which is
taken into account for testing UHV equipment (Gockenbach 2011; Okabe et al.
2013). It is expected that the upper tolerance limit for testing UHV equipment
(Vm [ 800 kV) will be in the order of 2.5–3.5 ls.
322 7 Tests with High Lightning and Switching Impulse Voltages
k-factor IEC 60060-1:2010
1
more compact insulation,
0.8 fast breakdown process
0.6
0.4 larger insulation,
slow breakdown
process
0.2
0 0.1 1 MHz 10
0.01
overshoot frequency f
Fig. 7.34 Expected modifications of the test voltage function
7.2.1.3 Interaction Between HVLI Test System and Test Object
Most test objects—like insulators, bushings, GIS, power transformers or cable
samples—provide a capacitive load for the test system. In few cases, test objects
have an inductive characteristic, like the low-voltage winding of power trans-
formers. Resistive test objects do not play any role, because outdoor insulations are
not tested under wet or polluted conditions with LI voltages. The influence of the
test object shall be explained by the equivalent single-stage circuit (Fig. 7.4).
Capacitive test objects: The load capacitance Cl = Cb ? Ci (usually consisting
of the basic load Cb of the generator plus the test object capacitance Cto) deter-
mines the front time constant sf (together with the front resistor Rf, Eq. 7.2), the
time constant for the tail st (together with the impulse capacitor Ci and the tail
resistor Rt) and the circuit efficiency factor gc [together with the impulse capacitor
Ci of the generator, Eq. (7.1)]:
Ci Á ðCb þ CtoÞ
sf ¼ Rf Cb þ Cto þ Ci ; ð7:22Þ
st % RtðCi þ Cb þ CtoÞ; ð7:23Þ
gc % Ci þ Ci þ Cto : ð7:24Þ
Cb
The front timeT1 (characterized by sf, Eq. 7.22) depends strongly from the test
object capacitance because usually Ci ) Cb ? Cto and consequently
sf & Rf Á (Cb ? Cto) with often Cto [ Cb. In many cases, the front time is nearly
doubled when the test object capacitance is doubled. Considering that the width of
the front time tolerance (0.84–1.56 ls) is less than doubling its lower limit, the
front resistors Rf of a generator must be adapted accordingly. For a fixed value of
7.2 Requirements to LI/SI Test Systems 323
(a) limitations by: (b) limitations by:
basic load Cb, basic load Cb,
tail
front front time tolerance, tail time tolerance
resistance overshoot
5% resistance
Rt
Rf T1 =1.56 s T2 =60 s
1000
800
Rf1 T1 = 0.84 s
500 700 Rt
200 Rf2 600 T1 = 40 s
100 500
0.1
0.5 1 5 nF 10 0.1 0.5 1 5 nF 10
total load capacitance Cb+Cto total load capacitance Cb+Cto
Fig. 7.35 Influence of the load capacitance on selection of front and tail resistors of a generator
2,000 kV/400 kJ of ten stages. a Principle of the selection of front resistance. b Situation of the
tail resistance
sf—respectively, of the front time T1—the necessary value of Rf for the upper,
respectively, the lower tolerance limit of the front time can be calculated (Kind
and Feser 1999) (Fig. 7.35a). A horizontal line through the resulting band corre-
sponding to a certain range of load capacitance gives the range of load, for which
one resistance Rf1 can be applied to deliver standard LI voltages. For higher load
capacitance, a lower resistance Rf2 has to be applied and so on. If the front
resistance becomes too low, an unacceptable over-shoot is generated which limits
the generation of standard LI impulses. This is reached when the front resistance
approaches about 100 9X (This means 10 9X per stage).
For the adaptation of the front resistance, each LI generator has several sets of
resistors. Usually, these sets are designed in such a way that resistors can also be
switched in parallel or even in series to vary the available values of front resistance
by combinations for a wide range of capacitive load. For each single-resistance
value, a certain load range is covered. Usually, a LI test system is equipped with
three sets of front resistors enabling seven different resistance values by different
parallel connections. For efficient handling of a larger LI test generator, the not-
used resistors should be stored on the stages which should be easily reached by
internal fixed ladders (Fig. 7.12c). Each impulse test system should be equipped
with an instruction which resistors should be applied for which test object
capacitance to cover a certain range of load capacitances (Fig. 7.36).
The time to half-valueT2 (characterized by st, Eq. 7.23) is not strongly influ-
enced by the test object capacitance because it is dominated by the impulse
capacitance Ci (Fig. 5.35b). One tail resistance is sufficient to cover the whole
tolerance band of the time to half-value between 40 and 60 ls.
324 7 Tests with High Lightning and Switching Impulse Voltages
Fig. 7.36 Selection of front limitation by Cb
resistance depending on test
object capacitance front time Tf Rf5 > Rf4 > Rf3 > Rf2 >
1.6 1.56 µs
µs
1.4
Rf1
1.2
1 increassing
front resistor
0.8
0 Rf
0.84µs
0.4 0.8 1. 2 1.6 nF 2.0
total load capacitance Cb + Cto
The circuit efficiency factor can easily be estimated by Eq. 7.24. The generator
with its basic load reaches usually a total efficiency factor g & 0.95 for Ci ) Cto.
It decreases slightly with increasing test object capacitance and values
Cto [ 0.2 Ci cannot be recommended.
An inductive test object combined with a capacitance is, e.g. found when the
low-voltage side of a power transformer is tested (Fig. 7.37a). The inductance Lto
forms an oscillating circuit especially with the impulse capacitance Ci and causes
an impulse tail which is characterized by an oscillation instead by an exponential
function. The oscillation shortens the time to half-value T2, often outside the
tolerance (T2 \ 40 ls), an undershoot to the opposite polarity as well as a
reduction of the efficiency factor (Fig. 7.37c). The influence on T2 is especially
serious and increases with decreasing impulse capacitance Ci of the generator
(Fig. 7.37c). As the LI test voltages of the low-voltage side of the transformer
under test are relatively low, several stages of a multi-stage generator are con-
nected in parallel to increase Ci. If this is not sufficient, the problem can be solved
by a so-called ‘‘Glanninger attachment’’: An inductance Lg is switched in parallel
to the front resistor Rf and a resistor Rg in parallel to the test object inductance Lto
(Fig. 7.37b). The Glanninger inductance Lg \ Lto is correctly selected when it
bridges the front resistor only for lower frequency (tail of the impulse) and when
the time to half-value is sufficiently extended, but the voltage is divided between
Lg//Rf and the test object (Lto//Rg) causing a further reduction of the efficiency
factor. The ‘‘Glanninger’’ inductance shall be in the order of Lg = (0.01–0.1) Lto,
and the ‘‘Glanninger’’ resistor is selected as Rg & Rf Á Lto/Lg. The Glanninger
attachment is a separate unit switched to the parallel connected stages of the
generator.
7.2 Requirements to LI/SI Test Systems 325
(a) (c)
Rf voltage Vt
Ci Rt (Cb+Cto) Lto Vt
(b) Lg circuit a) circuit b)
Rf time t
Ci Rt (Cb+Cto) Lto Rg
Vt
undershoot
Fig. 7.37 Testing of objects with inductances. a Equivalent circuit. b Equivalent circuit with
‘‘Glanninger’’ attachment. c Comparison of the impulse shape
7.2.2 SI Test Voltages
7.2.2.1 Requirements of IEC 60060-1 and IEEE Std. 4
Over-shoot is not a problem for SI test voltages because a front resistor of quite
high resistance is necessary to meet the front time parameter. The recorded curve
has also a sharp beginning, the so-called ‘‘true origin’’, and does not require a
virtual origin (Fig. 7.38). Therefore, the parameters are evaluated from the
recorded curve directly. When an SI voltage fulfils the following requirements, it is
a standard SI test voltage:
The test voltage valueVt is the maximum value of the recorded voltage curve
(Fig. 5.39). In a SI voltage test, the required test voltage value must be adjusted
with a tolerance of ±3 %.
The time-to-peakTp is the time interval between the true origin and the maxi-
mum value of an SI voltage. It replaces the front time T1 at LI voltages. The
time-to-peak is determined also from the time of the intersection TAB = t90 -
t30 (Fig. 7.39a) according to
TP ¼ K Á TAB ð7:25Þ
with K ¼ 2:42 À 3:08 Á 10À3TAB þ 1:51 Á T2
and Tp; TAB and T2 are in microseconds ðlsÞ:
326 7 Tests with High Lightning and Switching Impulse Voltages
time-to peak time-to half value
(2831 ± 22) µs
tolerance accepted range
reached variation
voltage (272.5±0.5) µs
2000
kV
1200
800
400
0
0 600 1200 1800 2400 3000 3600 µs 4800
time
Fig. 7.38 Recorded curves of SI test voltages 250/2,500 and their reproducibility for different
peak values
A standard SI test voltage requires Tp = 250 ls with a tolerance of ±20 %. This
means the real front time has to be within (200–300) ls.
Note When SI testing was introduced in the late 1960s, the evaluation of the time-to-peak
has been made according to all appearances. This principle has not been suitable for
computer evaluation, and therefore, the evaluation based on the intersection has been
introduced with IEC 60060-1:2010.
The definition front time—which would be in the order of T1 & 170 ls—has not been
introduced because the equivalent time-to-peak of Tp = 250 ls is used for decades, and
for traditional reasons it should not be changed.
The time to half-valueT2 is a virtual parameter as the time interval between the true
origin, and the instant when the voltage crosses the half of the test voltage value
(Fig. 5.39a): It is required T2 = 2,500 ls with a tolerance of ±60 %, this means
the real value has to be within (1,000–4,000 ls).
Note The tremendous tolerance of T2 is related to tests on equipment with saturation
phenomena, e.g. caused by the iron cores of transformers and reactors. There the SI
voltages cause a saturation of the magnetic core which leads to the immediate collapse of
the voltage (Fig. 5.39b). For such tests additional parameters, the time above 90 % and the
time to zero have been introduced (see below). The wide tolerance interval is acceptable
because the breakdown process in air—development of leader discharges (see Fig. 7.3)—is
determined by the steepness of the front of the SI voltage and very few by development of
its tail.
7.2 Requirements to LI/SI Test Systems 327
Fig. 7.39 Definitions of SI (a) time t
test voltages. a Standard 90%
impulse 250/2,500. b SI normalized voltage V/Vp
testing of equipment with time
saturation effects 1 peak value Vp
0.9
TT90
T
time above 90%
0.5
0.3
0
TAB
time-to-peak Tp = K·TAB
time-to-half value T2
(b)
voltage V/VP
100
%
80
time above 90%
60
40
20
0
time to zero Tz
The time above 90 %T90 is the interval during which the SI voltage exceeds 90 %
of its maximum value (Fig. 7.39b).
The time to zeroTZ is the interval between the true origin and the instant when the
SI voltage has its passage through zero (Fig. 7.39b).
The parameters shall not been mixed up, because a standard SI test voltage 250/
2,500 has to be characterized by the set (Vt; Tp; T2) and for testing equipment with
saturation effects the different SI set (Vt; Tp; T90; TZ) may be used.
7.2.2.2 Interaction Between HVSI Test System and Test Object
Consideration of polarity effects: Both, LI and SI voltage tests on internal insulation
shall be performed at negative polarity to avoid flashovers at bushings or termi-
nations in air. Whereas non-uniform air insulations have much lower breakdown
voltages at positive polarity, there is approximately no, sometimes even a slight
328 7 Tests with High Lightning and Switching Impulse Voltages
opposite polarity effect for internal insulation. Also the design of the control
electrodes and of the insulation structures of the HV components of an LI/SI test
system is determined by the maximum positive SI voltage to be generated. Positive
specific SI breakdown voltages of non-uniform fields in air can go down to the order
of 1 kV/cm, whereas at LI voltage one can assume a value of 5 kV/cm.
Capacitive test objects: It has been shown in Sect. 7.1.2 that the efficiency
factor can be understood as the product of the shape efficiency and the circuit
efficiency (Eq. 7.1). The shape efficiency factor gs of SI voltage is much lower than
of LI voltage. The circuit efficiency factor gc can be estimated according to
Eq. 7.24 as for LI voltages. For a load capacitance Cl = Cb ? Cto = 0.2 Á Ci, the
circuit efficiency becomes gc = 0.83. With a shape efficiency of gs = 0.75 one
gets a total efficiency factor of only g = 0.62 (Eq. 7.3). The time-to-peak is less
sensitive to changes of the load capacitance than the front time of LI voltage. The
tolerance of the time to half-value is so large that changes of the test object do not
play any role for the selection of the tail resistors. When an LI/SI test system is
ordered the maximum expected test object capacitance must taken into consid-
eration for the selection of the impulse energy per stage (Eq. 7.3).
Resistive test objects: Wet and pollution tests of external insulation are also
performed at SI test voltages, especially for EHV and UHV equipment. The
influence of the resistance of a polluted test object to the total front resistance of an
SI generator (up to some 10 k9X) remains negligible. But during testing heavy
discharges with currents of some Amperes may cause remarkable voltage drops.
This is not only a problem for wet and pollution tests, but also when long air gaps
with leader discharges are investigated. As described above for AC and DC
voltage, the voltage drop can be calculated when the height and duration of the
current pulse are known or assumed. Impulse currents of peak values above 10 A
and duration of some 10 ls have been observed (Les Renardieres Group 1977).
There is not yet any standard on the acceptable voltage drop at a pre-given current
impulse. In any case for the mentioned tests, an impulse generator of high impulse
energy should be applied.
Inductive test objects with saturation effects: For testing power transformers,
the IEC 60076-3: 2013 requires Tp [ 100 ls; T90 [ 200 ls and TZ [ 1000 ls. For
the prolongation of the time to zero, a pre-magnetization of the core with SI
voltage of opposite polarity and lower maximum value (Ve [ 0.7 Á Vt) can be
performed if necessary. Also after a SI voltage test, the core should be demag-
netized by applying lower SI voltages of opposite polarity.
7.3 Procedures and Evaluation of LI/SI Voltage Tests
Breakdown and standardized withstand voltage tests as well as the statistical
background are already described in Sect. 2.4. The following explanations are
related to special LI/SI test procedures and refer to that section.
7.3 Procedures and Evaluation of LI/SI Voltage Tests 329
7.3.1 Breakdown Voltage Tests for Research
and Development
The multiple-level method (MLM) is the most important and mainly used test
method for the determination of the performance function of insulation samples
(see Sect. 2.4.3 and Fig. 2.31). The performance function describes the relationship
between stressing LI/SI voltages and breakdown probabilities completely. The
MLM test can be easily performed on self-restoring insulation in air. The inde-
pendence might be checked by independence tests. A procedure for that test is
explained in Table 2.8. Usually the test results for air gaps are independent when
the break between two LI/SI voltages is not shorter than several seconds.
For air or SF6 insulation with surfaces to solid insulation (e.g. insulators), the
check of independence is more important than for gases alone before the further
evaluation of the test. If dependence is indicated, the test procedure should be
modified, e.g. by longer breaks between two impulses following each other. Also
the short application of a lower impulse voltage of opposite polarity or of a low
AC voltage during the break may help to get independent results.
For liquid impregnated and solid insulation the MLM application requires for
each LI/SI voltage stress a new test sample. This is a remarkable effort and the
limited reproducibility of the test samples causes an additional contribution to the
dispersion of the measured performance function. For such test objects it can be
checked to apply the progressive stress method (PSM; Fig. 2.26c). where one
sample is applied to a series of impulses until breakdown. A single test sample
delivers more information in a PSM test than in a MLM test.
The statistical evaluation should be made with a powerful software package
based on the maximum-likelihood method (Speck et al. 2009). This delivers
(Fig. 7.40) estimations of the breakdown probabilities including confidence
regions for the used seven voltage levels, point estimations of the performance
function (red line in the middle), its confidence limits (blue limits) and also
confidence limits of the quantiles (pink lines). Figure 7.40 shows the evaluation
based on the normal (Gauss) distribution. It can be repeated for a different dis-
tribution function to find the optimum adaptation. All possible conclusions can be
drawn from a diagram like Fig. 7.40.
When instead of the whole performance function, the estimation of a certain
quantile (e.g. V50 or V10) is sufficient, the up-and-down method can be applied
(Sect. 2.4.4 and Figs. 2.38 and 2.39). Also for that test procedure a maxi-
mum-likelihood evaluation is possible and can be recommended.
7.3.2 LI/SI Quality Acceptance Tests
A passed quality acceptance LI/SI test verifies the insulation coordination (Sect.
1.2) by performing the test according to a standardized procedure, see Sect. 2.4.6
and IEC 60060-1:2010.
330 7 Tests with High Lightning and Switching Impulse Voltages
breakdown confidence limit best estimation of
probability of distribution function of quantile performance function
99 quantiles:
98 97.7% 50 + 2
95
90 50 +
%
70
50 point and 50
30 confidence 50
20 estimation of 50
10 probability
1130 1150 1170
5 1190 voltage / kV
2
1
1110
Fig. 7.40 Performance function of an air gap evaluated by the maximum-likelihood method
based on a normal (Gauss) distribution
For external, self-restoring insulation (mainly of atmospheric air), two proce-
dures are acceptable (see also 2.4.6):
A1. The LI/SI test voltage is lower than the 10 %—breakdown voltage: Vt \ V10.
A2. The LI/SI test voltage is 15-times applied and the number of breakdowns is
k B 2.
For internal, non-self-restoring insulation (all solid or liquid impregnated
insulation) no breakdown may occur during the quality acceptance test; therefore,
the following procedure is applied:
B. The LI/SI test voltage is three times applied and no breakdown may occur
(k = 0).
For insulation as that of a GIS consisting of a self-restoring (SF6 gas) and a non-
self-restoring (solid insulators) part, it has to be shown that the breakdown had
happened in the self-restoring part. This can be indicated by a certain number of
withstands at the LI/SI test voltage after the breakdown. A further indication can
be a PD measurement which shows no increased PD level. The valid procedure is
specified by the relevant apparatus committee of IEC or IEEE. In the following
some examples for LI/SI acceptance test procedures are given:
Testing of external insulation (IEC 60071-1): The procedure A1 (Vt \ V10)
requires the estimation of V10 from a performance function measured down to this
low breakdown probability or for known V50 and standard deviation r from
7.3 Procedures and Evaluation of LI/SI Voltage Tests 331
V10 = V50 - 1.7 r or from an up-and-down test with seven impulses per voltage
level (see Sect. 2.4.4). The procedure A2 (n = 15/k = 2) is applicable for all other
cases when the above necessary parameters are not available and the effort for
their determination is assumed to be too high. It is necessary to mention that the LI
voltage tests are applied to dry insulation (see Sect. 2.1.2), whereas the SI voltage
tests are applied to wet insulation (see Sect. 2.1.3).
Testing of gas-insulated substation (GIS) (IEC 62271-203:2003): The GIS insu-
lation is characterized by a self-restoring part, the SF6 gas, and a non-self-restoring
part, the surface and the epoxy resin of the spacer insulators. The LI/SI voltage tests
are combined with numerous dielectric, thermal, power and mechanical measure-
ments and tests. The standard waveforms are applied and the procedure A2 (15/2)
shall be applied. It has to be guaranteed that no breakdown or flashover occurs in the
non-self-restoring part. This is considered as verified, if the last five impulses are
without such a disruptive discharge. If the first breakdown appears after the impulse
no. 10, the number of total impulses has to be extended accordingly. In the worst case
of a breakdown at no. 15, the total number of test impulses becomes 20.
Note It is important to state that the ‘‘horizontal’’ IEC 60071-1:1993 requires only three
impulse voltages and in case of one breakdown nine additional voltage impulses during
which no disruptive discharge is tolerated. This procedure is of higher uncertainty, and
therefore, it had not been considered as sufficient when the ‘‘vertical’’ GIS standard was
established in 2003.
Testing of power transformers (IEC 60076-3/FDIS:2013): The used impulse
generator shall exceed a minimum impulse energy. The standard recommends an
energy Wi min (in joules) according to the empiric formula:
ÀÁ
100 Á 2pf Á T22 Vt2 Á Sr ;
Wi min [ ZÁ Vm2 Á g2 ð7:26Þ
with the parameters of the transformer under test:
Vm rated phase-to-phase voltage in volts;
f rated frequency in Hertz;
Z short-circuit impedance in % related to the test terminals;
Sr three-phase power rating in Volt-Amperes;
and with the test parameters:
Vt LI test voltage value in volts;
T2 time to half-value of he LI voltage in ls;
g efficiency factor in per unit.
The LI voltage test is a routine test for power transformers Vm [ 72.5 kV, for
lower rated voltages a type test. For transformers Vm [ 170 kV, the test includes
also chopped LI voltages (LIC). An SI voltage test is a routine test for transformers
Vm [ 170 kV and for lower rated voltage a special test. The impulse shall be of
332 7 Tests with High Lightning and Switching Impulse Voltages
standard shape 1.2/50 within the usual tolerances. The evaluation according to the
k-factor method is allowed, but alternatively IEC 60076-3: 2013 defines some
strange differences to IEC 60060-1:2010:
As long as the over-shoot does not exceed 5 % (ß B 5 %), the extreme value
may be taken as the test voltage value. If the over-shoot exceeds 5 %, the front
time might be extended up to T1 = 2.5 ls and a test with chopped lightning
impulses must be performed. Remains ß B 5 % also now, the test voltage value is
the extreme value. Only for rare cases, when the over-shoot ß [ 5 % cannot be
avoided, the evaluation according to IEC 60060-1:2010 shall be applied for
transformers of rated voltage B800 kV. For UHV transformers, even longer front
times can be agreed between the parties of an acceptance test. Also the lower
tolerance limit of the time to half-value can be reduced up to T2 = 20 ls by
agreement. The test shall be performed in the following sequence:
• one full LI reference voltage of (0.5–0.6)ÁVt,
• one full LI test voltage Vt,
• two chopped (LIC) test voltages of 1.1 Vt,
• two full LI test voltages Vt .
If an SI voltage test is performed, it follows after the LI/LIC test and before the
tests with AC voltages (see Sect. 3.2.5). It consists of
• one SI reference voltage (0.5–0.7) Vt and
• three SI test voltages Vt.
Both tests are successful if no internal breakdown collapses the voltage. For the
LI test, additionally, the normalized voltage shapes of the reference LI voltage and
of the LI test voltage, as well as the normalized shapes of the measured impulse
currents (see Sect. 7.5) at the two voltage levels should be identical.
LI/SI testing of cables (IEC 62067:201): The routine test of cables does not
include an LI/SI test voltage. The AC withstand test and a sensitive PD mea-
surement are considered to be sufficient for verifying a correct production, but in
defined intervals, more detailed cable sample tests are performed to confirm the
correct production. A type test includes a long set of single tests including LI/SI
tests which are performed at warm cable samples after a heating cycle voltage test,
which includes 20 single cycles of one day each, and a PD measurement. First, the
SI test is performed with 10 positive and 10 negative SI voltages. If no breakdown
occurs, the sample has passed and can be stressed with LI voltages according to the
same procedure. The PD test is repeated after the LI/SI test.
Additionally, there are pre-qualification tests on complete cable systems of
about 100 m length with joints and terminations. The total test duration is about
one year with in minimum 180 heating cycles. The LI test voltage shall be applied
to the whole test assembly or to samples with a length of in minimum 30 m. The
test temperature shall be in an interval between maximum conductor temperature
and 5 K above. The test consists of 10 positive and 10 negative LI voltage
applications again. It is passed if no breakdown occurs.
7.3 Procedures and Evaluation of LI/SI Voltage Tests 333
As the capacitances of the cable samples are between 0.15 and 0.3 nF/m, the
test object capacitance for 30 m samples may reach 9 nF and for 100 m up to
30 nF. These are quite high capacitances, and therefore, the standard allows front
times T1 = 1…5 ls, T2 is within the standard values T2 = 40…60 ls. For SI test
voltages, the usual tolerances shall be applied.
7.4 Measurement of LI and SI Test Voltages
For measuring the peak value of high impulse voltages originally, sphere gaps
have been used, as already presented in Sect. 2.3.5. This direct measurement
method, however, is nowadays only recommended for performance checks as well
as linearity tests. Occasionally, also field probes described in Sect. 2.3.6 are
applicable, particularly for measurement of fast transient voltages characterized by
front times substantially below the ls range (Feser and Pfaff 1984). This sub-
section deals only with indirect measuring methods using a converting device and
a measuring instrument both connected via a transmission system. As the design of
the converting device is the most challenging task, particularly if intended for LI
voltage measurements, the following treatment focuses mainly on this topic. In this
context, it should be noted that more details on the fundamentals, instrumentation
and procedures for LI voltage measurements can be found in a textbook of Schon
published in 2010.
7.4.1 Dynamic Behaviour of Voltage Dividers
Lightning impulse voltages, particularly if chopped in the front, cover a frequency
spectrum up to more than 10 MHz. To prove whether the scale factor remains
constant within such a wide frequency band, the dynamic behaviour of the mea-
suring systems applied must be known. As the transfer function is mainly governed
by the voltage divider providing the converting device, only this component shall
be investigated in the following. Basically, the transfer function can be determined
in both the frequency and the time domain. A set-up used for the second method is
sketched in Fig. 7.41.
Usually, a step voltage of several 100 V having a rise time in the order of one
nanosecond is applied. The short rise time is accomplished by means of a mercury-
wetted relay, often referred to as Reed relay. If switched at repetition rate around
100 Hz, disturbing background noises can effectively be rejected if the recording
device is equipped with a feature for signal averaging. Due to the proximity effect,
it has to be taken care that the voltage divider is arranged in agreement with real
HV test conditions where neither the HV connection lead nor the measuring cable
should be replaced after the performance test has been finished.
334 7 Tests with High Lightning and Switching Impulse Voltages
reed relay damping resistor HV connection lead
R1 converting device
measuring instrument
HV resistor
load resistor transmission system
DC voltage source V1 R2 V2
LV resistor
Fig. 7.41 Set-up for measuring the step voltage response of LI voltage dividers
Among others, the dynamic behaviour is mainly affected by the stray capaci-
tances between divider column and earth as well as other grounded structures. For
a better understanding, consider the equivalent circuit of a resistive voltage divi-
der, as shown in Fig. 7.42a. Here, the HV arm is subdivided in n equal elements
where the partial resistors and earth capacitance are linearly distributed:
R11 ¼ R12 ¼ Á Á Á ¼ R1n ¼ R1=n; Ce1 ¼ Ce2 ¼ Á Á Á ¼ Cen ¼ Ce=n:
For such a network, the potential distribution along the HV divider column is
given by a hyperbolical function equivalent to that of long transmission lines
(Raske 1937; Elsner 1939; Asner 1960). In the following, it shall be assumed that
the resistance R1 providing the HV arm is much greater than the resistance R2
providing the LV arm, which is always satisfied for HV dividers. Under this
condition, the voltage v2 (t) appearing across R2 can be deduced from the voltage
v1 (t) applied to the top electrode using the Laplace transformation. For the net-
work with distributed elements according to Fig. 7.42a, one gets
FðjxÞ ¼ V2ðjxÞ % R2 Á sin hðcÞ ; ð7:27Þ
V1ðjxÞ R1 sin hðn Á cÞ
with
ðn Á cÞ2¼ jx Á R1 Á Ce: ð7:28Þ
To keep the measuring uncertainty as low as possible the condition, ðn Á cÞ2( 1
must be satisfied. Under this condition, Eq. 7.27 can be simplified as follows:
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High Voltage Test and Measuring Techniques By Wolfgang Hauschild and Eberhard Lemke
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